First, batting (sometimes called by the brand name Dacron) reduces the friction foam has, and thus reduces wear to fabric.
Second, because polyester batting remains springy, it is ever ready to put some light pressure against fabric. This means that even as the fabric stretches with age (and always happens) batting will push against the fabric and keep wear-worn waves from developing.
And thirdly, and most importantly, batting fills in any slackness in how you've upholstered your furniture, cushion, etc. And because this will make your upholstered piece looks better.
Adding poly-bonded Dacron to replacement cushions will make a huge difference in the look and feel.